This disclosure is directed to a method and apparatus useful in determining the rate of corrosion. Corrosion occurs in pipelines made of metal. The corrosion which occurs is a function of many factors. In an effort to counteract the rate of corrosion, corrosion inhibitors are injected into the flowing material in the pipeline. It is important to get the right amount of inhibitor material in the pipeline. If insufficient material is injected, inadequate protection will be achieved and corrosion may nevertheless still occur. By contrast, if excessive corrosion inhibitor material is injected, it may be wasted, and it is usually an expensive additive. For these reasons, it is desirable to determine the right rate at which the injection material is injected. This particular situation has a long protective term aspect which enables the metal forming the pipeline to be protected and to extend the life of the pipeline. The rate of corrosion and hence the severity of corrosion increases with a number of factors. However, these increases are not necessarily linear.
It is intended that a corrosion inhibitor be injected at a required volume taking into account the flow rate so that the corrosion of metal can be substantially reduced. The inhibitors which are added significantly interact with the wall of the pipeline. At the wall, there may well be a "coated" metal surface. The surface is normally coated by reaction products which are obtained from partial corrosion, that is, intermediates of the corrosion process. Sometimes, the corrosion process is relatively complicated and forms numerous intermediates. Take as an example a simple situation, namely, one in which a flowing liquid product includes water and is flowing in a steel pipeline. The water will react with the wall of the pipe and form rust meaning various oxides of the iron in the steel pipe. This will form a highly undesirable rust layer. Clearly, the thickness of the layer will vary and the precise mix of the oxides of iron will likewise vary. Intermediates may be formed which are different types of iron oxide. Furthermore, the layer construction may vary i.e., it may be open or porous so that other products can penetrate through the surface material and still react with the metal layer beneath the corrosion layer. Corrosion layers typically are accumulated to the detriment of the pipeline. In many instances, it may be important to achieve an impervious layer on the wall of the pipe. The inhibitor is added to the flowing pipeline product so that the inhibitor product reacts with the flowing product and the surrounding pipeline to thereby form a coating which inhibits further chemical reaction with the metal forming the pipeline. It is especially important to screen the steel of the pipeline from sulfur related products and especially H2S. H2S has a propensity for severe damage to a pipeline. In one aspect of the present invention, coating is assumed to be formed on the interior of the pipeline, and with regard to the coating, the present system enables measurements to be made.
The rate of corrosion depends on very complex factors. It is highly desirable to know the rate of corrosion so that protective steps can be taken. In one aspect of the present invention, a flow loop is set forth which utilizes a closed loop having a high pressure pump connected in the loop to thereby provide fluid flow at a specified velocity. It has been learned that the rate of corrosion is dependent on several significant factors, and others may be applicable in other circumstances. One important factor is the temperature of the flowing material. While the product may be substantially inert, as the temperature is increased, it becomes a more reactive material in ordinary circumstances. The rate of corrosion is also dependent on the velocity of the flowing liquid. Fluid traveling at an elevated velocity changes the manner or mode in which the flowing fluid reacts with the wall of the pipe. The inhibitor may be flushed away from the wall of the pipe. When this occurs, substantial protection which would otherwise be present at low velocities is lost and may not be available at high velocities.
Not only do temperature and velocity have an impact on the flow rate, but the corrosion rate is also dependent on ambient pressure. In other words, for a given set of circumstances, if the pressure were increased by 100%, the corrosion rate is changed as a result of that interaction.
As will be understood, the examples of pressure, temperature and velocity are simply three of the several variables which interact to describe the corrosion rate. Even more profoundly, the corrosion rate still must be evaluated with respect to other factors. Suffice it to say, the rate of corrosion is a relatively complex relationship and is dependent on the three mentioned factors and also on other factors and can be generally described as a nonlinear relationship.
The present disclosure is directed to a method and apparatus for determining the flow rate of corrosion. More particularly, the present disclosure sets forth a system which includes a test loop for providing controlled fluid flow where the above mentioned three physical factors can be varied. That is, the system is able to vary pressure, flow rate, and temperature. In this aspect, the test loop utilizes suitable storage tanks which furnish the necessary test fluid. As desired, a gas drive can be applied behind the system to assure that the pressure is brought up to the requisite test range. Coupled with this, the system also contemplates the use of multiple lines cooperative with individual test cells to obtain different corrosion rates for differing flow rates. More specifically, multiple test cells are arranged and deployed in conjunction with test instrumental which provides needed measurements. The corrosion cell utilizes metal coupons which are exposed to the fluid flow. The metal coupons in the preferred embodiment are brought into contact with the flowing liquid. In particular, the flowing liquid is exposed so that the surface is necessarily impacted by the corrosion inhibitors when added to the fluid flow, and a corrosion rate is determined from that. Utilizing a potentiostat, the rate of corrosion on these steel coupons can be determined. In addition to that, the rate of corrosion is also measured through the use of a variable frequency oscillator (VFO) to provide a sweep frequency signal. This is done in conjunction with a pair of test coupons to provide an indication of the nature of the corrosion, and which is subject to later interpretation to provide some indication of the rate of corrosion buildup, the rate of protection, and the ability of the corrosion inhibitor to prevail notwithstanding extraordinary high pressure, flow velocities, and temperatures. Other variables may be adjusted also and data regarding them is obtained so that the indications of the corrosion rate can be correlated to any number of variable factors. As an example, one variable factor is the presence of H.sub.2 S in the flowing liquid which has its own particular impact on the corrosion rate.